Portable gas spectroscopy may be useful in settings that include indoor or confined space air monitoring and breath analysis. Current technologies include either small devices that detect a limited number of gas types (such as for common roadside alcohol breathalyzer tests) or may lack sensitivity. Larger devices may be able to scan for a broader number of gas types, but their size and power consumption may make them undesirable in many environments.
As an overview of the technology, electro-magnetic waves in the millimeter and submillimeter wave frequency ranges can be used for fast scan rotational spectroscopy to detect gas molecules and measure their concentrations. As an overview of how rotational spectroscopy works and as shown in FIG. 1A, a transmitter 50 such as a VDI transmitter (using components from Virginia Diode Inc, manufacture of high frequency GaAs devices) radiates electromagnetic waves 55 within a certain frequency range into an absorption cell 60 containing gas molecules 65. The waves change the rotational state of gas molecules and a part of the wave power is lost in this interaction. This results in a drop of power picked up 75 at a receiver (RX) 70 such as a VDI receiver or a spike. By sweeping the transmitter (TX) 50 output frequency, a sweep controlled by a computer (not shown), the receiver 70 may measure the frequency response of the absorption cell 60. As shown in the frequency vs power absorption graph 90, looking at the frequencies at which the waves are absorbed and the depth of absorption line, the presence of a particular molecule and its concentration may be determined.
Rotational spectrometers use a frequency multiplier chain driven from a signal generated using a synthesizer to generate the transmitter signal, and a diode based sub-harmonic mixer followed by an amplifier and a diode amplitude detector to measure the received signal amplitude. The frequency multiplier chain, mixer and diode may be fabricated using compound-semiconductor technologies. Rotational spectroscopy at 240-250 GHz has been demonstrated using SiGe heterojunction bipolar transistor (HBT) based radio frequency (RF) front-ends for transmission and reception and a spectrometer that measures 200-300 GHz frequency waves uses III-V technology (obtained by combining group III elements (Al, Ga, In) with group V elements (N, P, As, Sb)) but is both expensive and bulky.
In use in a wideband transmission/detection scenario, a transmitter for the spectrometer may generate an FM signal that can be scanned over ˜100-GHz frequency range with a 10-kHz step. The transmitted power level may be −30 to −10 dBm to avoid the saturation of gas molecules in a sample.
FIG. 1B shows one such doubler-based transmitter 100. The doubler based transmitter includes a fractional phase locked loop (PLL) 110 that generates a 90 to 150 GHz signal with −20 to −10 dBm transmitted power level. A Marchand Balun 120 received this PLL signal and generates a differential signal that the wideband amplifier 130 transmits to the driver 140 that transmits the 90-150 GHz signal to the doubler 150, which outputs the signal as a 180 to 300 GHz signal with a >−10 dBm power level.
This achieves the target frequency range, but at a cost. As shown in FIG. 3, which is a conceptual diagram depicting output power vs input power response for mixer and doubler based systems, for a moderate output power requirement, the input power required for the doubler based design requires a higher output than the mixer design (to be introduced hereafter). Also, the doubler based system generates twice the frequency of input signal. If the input signal is not able to cover the complete frequency range, the output will not be continuous. The wideband PLL at high frequency especially the one using magnetic switching may be prone to frequency gaps in their operating range.